An electrophotographic developing method is a method of developing an electrostatic latent image formed on a photosensitive body by adhering thereto a toner particle in a developer, and the developer used in this method is classified into a two-component developer composed of a toner particle and a carrier particle, and a one-component developer using only a toner particle.
As the developing method using, out of these developers, a two-component developer composed of a toner particle and a carrier particle, a cascade method, etc. have long been employed, but a magnetic brush method using a magnet roll is currently the mainstream.
In a two-component developer, the carrier particle is a carrier substance which is stirred together with a toner particle in a development box filled with the developer to impart a desired charge to the toner particle and furthermore, transports the charged toner particle to the surface of a photoreceptor to form a toner image on the photoreceptor. A carrier particle remaining on a magnet-holding development roll is again returned to the development box from the development roll, mixed/stirred with a fresh toner particle, and used repeatedly for a given period of time.
In a two-component developer, unlike a one-component developer, the carrier particle is mixed/stirred with a toner particle to exert a function of charging the toner particle and transporting the toner particle and has good controllability when designing a developer. Therefore, the two-component developer is suitable, e.g., for a full-color development apparatus requiring high image quality, or an apparatus of performing high-speed printing, in which reliability and durability in image preservation are required.
In a two-component developer used in this way, it is necessary that image characteristics such as image density, fogging, white spot, gradation and resolution show predetermined values from the initial stage and moreover, these characteristics are stably maintained with no variation during endurance printing. In order to stably maintain these characteristics, the properties of the carrier particle contained in the two-component developer must be stable.
As the carrier particle forming a two-component developer, various iron powder carriers, ferrite carriers, resin-coated ferrite carriers, magnetic powder-dispersed resin carriers, etc. have been conventionally used.
With the recent progress of office networking, the age of monofunctional copier evolves into the age of multifunctional copier, and the service system is also shifted from the age of system where a contracted service man performs periodic maintenance inclusive of replacement of a developer, etc., to the age of maintenance-free system, as a result, the market demand for a further longer life of the developer is more increasing.
Under these circumstances, in Patent Document 1 (JP-A-H5-40367), etc., magnetic powder-dispersed carriers containing a resin having dispersed therein fine magnetic microparticles have been proposed with the aim to reduce the weight of the carrier particle and extend the developer life.
Such a magnetic powder-dispersed carrier can reduce the true density by decreasing the amount of the magnetic microparticle and in turn, can reduce the stress due to stirring, so that abrasion or separation of the coating can be prevented and stable image properties can be obtained over a long period of time.
However, in the magnetic powder-dispersed carrier, a magnetic microparticle is hardened with a binder resin, and there may arise a problem that a magnetic microparticle is detached due to a stirring stress or an impact in a developing machine or the carrier particle itself is broken, may be because the mechanical strength is poor compared with the conventionally employed iron powder carrier or ferrite carrier. The detached magnetic microparticle or the broken carrier particle attaches to a photoreceptor and gives rise to generation of an image defect.
Furthermore, the magnetic powder-dispersed carrier uses a fine magnetic microparticle and therefore, has a drawback that the residual magnetization and coercive force are increased and in turn, the flowability of the developer is deteriorated. In particular, when a magnetic brush is formed on a magnet roll, because of high residual magnetization and high coercive force, the ear of the magnetic brush becomes hard, and a high image quality can be hardly obtained. In addition, there is a problem that even when the carrier leaves the magnet roll, the carrier is not disaggregated from magnetic aggregation and fails in quickly mixing with a toner replenished and therefore, the rise of the charge amount is poor, causing an image defect such as toner dusting or fogging.
As a carrier to replace such a magnetic powder-dispersed carrier, a resin-filled ferrite carrier where a void in a ferrite carrier core material using a porous ferrite particle is filled with a resin, has been proposed.
Patent Document 2 (JP-A-2006-337579) has proposed a resin-filled ferrite carrier obtained by filling a ferrite carrier core material with a resin, where the void ratio is from 10 to 60%, and Patent Document 3 (JP-A-2007-57943) has proposed a resin-filled ferrite carrier having a sterically laminated structure.
These resin-filled ferrite carriers proposed by Patent Documents 2 and 3, etc. are advantageous in that the specific gravity is low to enable weight reduction, the durability is excellent, making it possible to extend the life, the strength is high compared with a magnetic powder-dispersed carrier and at the same time, the carrier is free from breakage, deformation and fusion due to heat or impact.
However, charge stability over a long period of time is required also for such a resin-filled ferrite carrier, and proposals therefor have been made. For example, Patent Document 4 (JP-A-2008-203476) describes a resin-filled ferrite carrier for an electrophotographic developer, obtained by filling a void of a porous ferrite core material with a silicone resin, wherein the average particle diameter is from 20 to 50 μm, (Si/Fe)×100 measured by fluorescent X-ray elemental analysis is from 2.0 to 7.0, the particle diameter is correlated with (Si/Fe)×100, and in the correlative relationship between [(Si/Fe)×100] and particle diameter, the gradient (a) of the correlation formula is −0.50≦a≦0.15. This resin-filled ferrite carrier is said to be advantageous in that so-called beads carry over is prevented and good charge amount stability is achieved, in addition to the above-described advantages of the resin-filled ferrite carrier.
Patent Document 5 (JP-A-2008-242348) describes a resin-filled ferrite carrier obtained by filling a void of a porous ferrite core material with a silicone resin, wherein the resin is a silicone resin having a softening point of 40° C. or more and being cured at a temperature not lower than the softening point and the filling amount of the resin is from 7 to 30 parts by weight per 100 parts by weight of the core material. This resin-filled ferrite carrier is said to be advantageous in that since the amount of a resin microparticle existing in the floating state without adhering to the porous ferrite core material is small, the developer produced comes to have stable charge characteristics and an image defect such as white spot is not caused, in addition to the above-described advantages of the resin-filled ferrite carrier.
Patent Document 6 (JP-A-2009-86093) describes a production method of a resin-filled carrier obtained by filling a porous ferrite core material with a resin, wherein a value obtained by multiplying the pore volume of a ferrite core material by the density of a filling resin is defined as a maximum filling amount (theoretical value) and the pore volume of the core material and the amount of the resin are set to afford a filling amount of 80 to 120% of the maximum filling amount. It is said that the resin-filled carrier obtained by this production method has a proper resin filling amount, allowing for no presence of a floating resin and in turn, leading to no generation of an image defect attributable to a failure in charging a toner or no generation of an image defect attributable to a low dielectric breakdown voltage, and at the same time, the carrier has high strength.
As described above, in Patent Document 4, Si/Fe is specified to determine the correlation with the average particle diameter, whereby the amount of a resin particle existing in the floating state is reduced and the charge stability, etc. are improved. In Patent Document 5, a specific silicone resin is used as the filling resin so as to stably obtain charge stability. In Patent Document 6, a value obtained by multiplying the pore volume of a core material by the density of a filling resin is defined as a maximum filling amount (theoretical value) and the pore volume of the core material and the amount of the resin are set to eliminate the presence of a floating resin.
In recent years, the pore volume of a porous ferrite particle used as a porous ferrite core material tends to be reduced. Because, not only the strength of the core material is increased and high durability is obtained, but also a decrease in the resin filling amount is afforded, which is economically advantageous. However, under such a circumstance involving reduction in the pore volume of a ferrite particle, it is difficult for the resin-filled ferrite carrier or the production method thereof described in Patent Documents 4 to 6 to afford a developer having good charge amount stability.
In addition, while the developer is required to have high durability and extend its life, a carrier having durability is also demanded and in turn, a weight-reduced carrier having a low specific gravity is demanded. Furthermore, the optimal specific gravity required of the carrier varies according to the system of the developing machine. In such a situation, a resin-filled carrier where only the true specific gravity can be arbitrarily designed while maintaining the characteristics of the resin-filled carrier is required. However, the resin-filled ferrite carrier or the production method thereof described in Patent Documents 4 to 6 cannot respond to such requirements.